SM-ND ISOTOPIC GEOCHEMISTRY OF SEDIMENTS FROM TAIWAN AND IMPLICATIONS FOR THE TECTONIC EVOLUTION OF SOUTHEAST CHINA

An isotopic and geochemical study on the Miocene to late Paleozoic sediments and metasediments of Taiwan has been conducted in order to decipher the crustal evolution of SE China. With the exception of a few late Paleozoic sediments, most samples from Taiwan have a narrow range in Sr and Nd isotopic ratios and SmNd crustal residence ages (tDM, assuming derivation from a depleted mantle source). The isotopic characteristics are similar to those of suspended material from major rivers around the world. In particular, they closely resemble those reported for the Yangtze River sediment and the pre-Cretaceous granitoids of SE China. No obvious trend in tDM or isotopic ratios has been found with respect to the depositional ages, indicating no pronounced secular variation. However, a few late Paleozoic samples might have been influenced by fresh mantle inputs to their provenance. Proterozoic tDM of the sediments combined with Proterozoic Pb components in zircons from Taiwan's granitoids suggest that important crustal growth in SE China took place in the middle Proterozoic. The isotopic and geochemical characteristics of the sediments suggest that their sources were, primarily, recycled pre-existing continental crust. REE contents of Taiwan sediments measured in this study are similar to the suspended load in major rivers, PAAS (Post-Archean Average Australia Shale), NASC (North American Shale Composite), and the continental shelf-slope sediments from the East China Sea off the mouth of the Yangtze. They seem to represent products derived from typical continental crust. Lower HREE abundances in Taiwan sediments, suspended load in major rivers, and surface sediments from the E. China Sea than those of PAAS suggest that the source of HREE in PAAS is likely to be slightly different from that for these recent terrigenous sediments. An alternative for the explanation of this HREE discrepancy is that a certain proportion of HREE in these recent sediments were left in the residue. Partially dissolution of heavy mineral in analysis procedures, particularly zircon, even in a Teflon ® bomb, might cause this HREE discrepancy. Late Yanshanian granitoids now widely exposed on the SE China coast have higher Nd and lower Sr isotopic ratios than those for Taiwan's sediments. So these granitic intrusions were not a major source for the sediments, indicating that they were not exposed to erosion until the Miocene or later. © 1990.